The invention relates to light emitting devices using periodic dielectric structures.
A very popular conventional light emitting device is the light emitting diode (LED). LEDs are p-n junction devices emitting spontaneous radiation in response to being forward biased. In the past decades, LEDs have come to play a critical role in numerous applications, including illuminations and flat-panel displays and optical communications. Although some conventional LEDs have high internal quantum efficiencies, i.e. high efficiencies of light generation inside the p-n junction, most emitted photons tend to be trapped inside the high refractive index semiconductor medium. As a result, the photons do not escape into free space, which leads to poor extraction efficiencies, typically less than 10%.
In the past thirty years, various approaches have been proposed to enhance the extraction efficiency of LEDs. Most of these techniques seek to enlarge the "escape cone" of photons. Ideally, the escape cone can cover the entire 4.pi. steradians when a point-like active region is placed in the center of a spherically shaped semiconductor device. This geometry allows light emitted from the active region to be incident on the semiconductor surface at an angle of 90.degree., resulting in the absence of total internal reflection losses. However, such spherical LEDs are incompatible with today's planar semiconductor technology. More practical geometries employ cubicle, cylinder or epoxy-dome geometries. Details of these structures can be found in E. F. Schubert, "GaAs Light-Emitting Diodes", Properties of GaAs, (INSPEC, Manchester, UK, 1996), incorporated herein by reference. However, none of these structures can entirely eliminate total internal reflection losses. Furthermore, reflection losses exist even for photons radiated inside the escape cone because of the refractive index mismatch between the LED chip and the surrounding media. While such losses could be reduced with an anti-reflection coating, complete cancellation of reflection occurs at only one frequency and one incidence angle.
Other approaches randomize the photon trajectory to enable multiple entry of photons into the escape cone. Trajectory randomization is achieved, for example, with photon recycling, or with surface texturing. As photons have to bounce back and forth several times before eventually entering the escape cone, their lifetimes are long, making these LEDs unsuitable for high-speed applications. Furthermore, as long photon lifetimes tend to increase parasitic losses, it is necessary to use high quality materials.
The conventional structures tend to rely on geometrical optical designs that do not alter the spontaneous emission properties of the devices. The first attempt to increase LED efficiency by direct modification of spontaneous emission was made by putting a quantum well active region in a microcavity defined with a silver reflector and a distributed Bragg reflector, and is described in Schubert et al., "Highly Efficient Light-Emitting Diodes With Microcavities", Science 265, 943 (1994), incorporated herein by reference. At resonance, the spontaneous emission along the axis of the cavity was strongly enhanced, leading to a higher external efficiency. However, off resonance, the emission was actually attenuated. Therefore, these resonant cavity LEDs could not provide enhancement over the entire emission spectrum.